This application claims the priority of Japanese Patent Application No. 2-160241 filed June 19, 1990, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to the composition of a gas generating agent for use in a gas generator for inflating an air bag.
2. Description of the Related Art
A folded air bag is installed in the steering wheel of a vehicle. When an accident occurs, gas is supplied to the air bag to inflate it, thereby protecting the driver and passengers.
The gas generator, which supplies gas to the air bag, retains a pellet of a gas generating agent consisting of, for example, an azide and an oxidant. Burning this gas generating agent generates nitrogen gas which inflates the air bag.
The gas generating agent must burn very quickly since the air bag should inflate within several tens of milliseconds after the ignition starts. If the burning rate of the gas generating agent is not high enough, the pellet of the gas generating agent are often made thinner to increase the burning surface area.
In addition, the amount of toxic gas released from the gas generator, such as carbon monoxide or cyanides, should be kept below a certain concentration or at the level which can not be detected.
As the pellet of the gas generating agent will inevitably experience violent vibrations and/or severe temperature conditions involving a significant temperature changes, the pellet should maintain a sufficient strength to endure such conditions.
It is known that the addition of a binder increases the strength of the pellets of the gas generating agent. Binders are generally classified into organic binders and inorganic binders. Various organic binders are, however, unsuitable for mixing with gas generating agents for the following reason.
The carbon component in organic binders is liable to cause the formation of carbon monoxide or cyanides during the burning process of the gas generating agent and thus generates toxic gas. Particularly, cyanides are extremely poisonous so that even a slight amount thereof should not be generated.
Various prior art has been developed in consideration of this point. Japanese Patent Publication No. Sho 56-13735 discloses a gas generating composition having a compound represented by the following general formula I, formulated into a gas generating agent containing an azide and an oxidant: EQU (Al.sub.2 O.sub.3).sub.m (M.sub.x O).sub.n (SiO.sub.2).sub.p.qH.sub.2 OI
where M represents Li, Na, K, Sr, Mg or Ca, x is either 1 or 2, m and n are 0 or a positive number (provided that m and n are not 0 at the same time); p is a positive number and q is 0 or a positive number.
As a specific example of the compound represented by the formula I, the Japanese publication discloses aluminum silicate, magnesium silicate, magnesium aluminate silicate, water glass and their combination. All of these compounds are synthesized products. This gas generating composition is effective at improving the strength of the pellet of a gas generating agent.
The gas generating composition disclosed in the Japanese publication has, however, a disadvantage in that the burning rate drops with an increase in the amount of the compound represented by the formula I added. This is apparent from the fact that the inflating time for the air bag, as disclosed in the publication, increases with an increase in the amount of the compound added.
When the burning rate is slow, inflating the air bag within a predetermined period of time requires that the pellet be made thinner to increase the burning surface area. This method, however, reduces the strength of the pellet. There is no advantage to adding an inorganic binder to improve the pellet strength if such a measure slows the burning rate.
Japanese Unexamined Patent Publication No. Sho 63-166427 discloses a gas generating composition containing an azide as a main component and 2 to 6% by weight of graphite fiber. More specifically, this document discloses the following composition:
______________________________________ sodium azide 61 to 68% by weight sodium nitrate 0 to 5% by weight bentonite 0 to 5% by weight iron oxide 23 to 28% by weight fumed silica 1 to 2% by weight graphite fiber 2 to 6% by weight ______________________________________
According to the composition, addition of the graphite fiber improves the pellet strength without reducing the burning rate.
Generally, in processing a gas generating composition into a pellet, a predetermined amount of a gas generating agent is supplied to the forming chamber of a commercially available tablet making machine and is compressed therein. At that time, in order to stably produce pellets with a given amount of chemicals and a given thickness, the gas generating agent before compression needs to have a fluid characteristic. In general, therefore, the gas generating agent is produced in the form of granules of the size of 0.1 to 1.0 mm.
Since the above composition has a fibrous material, such as graphite fiber, added, the gas generating agent cannot have good flowability. This makes it difficult to provide the desired granular form, resulting in an undesirable reduction of the working efficiency in producing pellets of a gas generating agent.
U.S. Pat. No. 3,996,079 discloses a gas generating composition having an azide, nickel oxide or iron oxide as an oxidant, and a small amount (0.5 to 3.0%) of a clay material. The clay material is effective in improving the efficiency of molding the gas generating agent.
Since this gas generating composition employs nickel oxide or iron oxide as an oxidant, the oxidizing reaction speed is slow, preventing the improvement of the burning rate.
U.S. Pat. No. 4,931,111 discloses a gas generating composition comprising (a) about 50to 70% by weight of an azide, (b) about 2 to 30% by weight of a first oxidant consisting of a metal oxide and (c) about 2 to 40% by weight of a burning rate controlling agent consisting of a second oxidant comprising nitrate or perchlorate, and a clay material (the ratio of the second oxidant to the clay material is 1:1 to 1:8). The burning rate of this composition is fast and gas generated from the burning has less toxicity.
This gas generating composition has a strong oxidant, such as nitrate or perchlorate, added thereto in order to acquire the desired performance. This causes a strong reaction to thereby raise the reaction temperature. In this case, the temperature of the generated gas becomes higher than what will result from the use of only a metal oxide as an oxidant. This requires that a cooling means be provided within the gas generator, so that the gas generator cannot be designed compact and lighter.
There is also a known composition which uses manganese dioxide as an oxidant to be mixed with an azide. However, a chemical property of azides is that when they are reacted with a heavy metal, such as copper, lead, silver and mercury, they are easily explosively ignited. Therefore, they are so sensitive that they should be treated with extreme care. The reaction of any azide with a heavy metal should therefore be avoided. Natural manganese ore, which is used as the raw material for manganese dioxide, contains a considerable amount of impurities, such as copper and lead. In order to mix the manganese dioxide acquired from the manganese ore with an azide, therefore, the manganese dioxide should be purified sufficiently to eliminate the heavy-metal impurity.
A typical method of refining manganese dioxide is to temporarily reduce it to manganese monoxide which is soluble in sulfuric acid, then selectively oxidize only the manganese monoxide in the bath of sulfuric acid. This purification process is preferred in that the heavy-metal impurity is eliminated to the degree of 10 ppm or below. The use of the sulfuric acid bath however causes the refined product to contain 4 to 5% of water, or adhesive water and bound water.
As a result, the composition having the manganese dioxide, produced through the above purification process, formulated into an azide has the disadvantage that it generates gas after burning, which contains a large amount of ammonia gas that has an bad odor while having a slight toxicity.